The human corneal epithelium consists centrally of five layers of squamous cells with a thickness in the range of 50 to 60 microns. Peripherally, the human corneal epithelium consists of eleven or more layers. With the onset of surgical treatments to the surface of the cornea, e.g., radial keratotomy, excimer laser shaping of the external surface of the cornea, etc., it has become vitally important to have an accurate map of thickness and contour variations of the various corneal layers.
Slit-lamp micropachymetry is presently the most widely used method for measuring the thickness of the corneal epithelium and corneal scarring. Such micropachymetry and other optical pachymetric methods suffer from a widely varying measurement reproducability, even in the hands of experienced users. For instance, precision for slit-lamp corneal pachymetry has been reported to vary between 5.6 and 19 microns for the same surface. Furthermore, optical methods exhibit the disadvantages of being limited to situations where the media is optically transparent--a condition not often present in areas of corneal scarring.
High frequency ultrasound scanning for imaging and pachymetry of the corneal epithelium was first Ultrasound Microscopic Imaging of the Intact Eye" Ophthalmology, Vol. 97, pages 224-250 (1990). In the Pavlin et al. system, measurements were made from B-scans and exhibited a loss of precision due to analog pre-processing of ultrasound signals, i.e., rectification and smoothing.
Lizzi et al. in "Theoretical Framework for Spectrum Analysis in Ultrasonic Tissue Characterization" Journal, Acoustical Society of America, Vol. 73 No. 4, pages 1,366-1,373 (1982) describe a clinical ultrasound system which employs deconvolution of a received signal to improve signal quality. Deconvolution, or inverse filtering, was used to compensate for system bandwidth limitations and acted to suppress out-of-band noise and to remove signal anomalies from a received echo. While deconvolution improved resolution of a received signal by removal of extraneous system-derived signals, it did not improve the distinctiveness of echo peaks. Such peaks are critical to accurately determining interfaces between various epithelial layers.
In 1981, Gammell introduced the use of an "Analytic Signal" into the processing of ultrasonic signals. (See "Improved Ultrasonic Detection Using the Analytic Signal Magnitude", Ultrasonics, March 1981, pages 73-76). Gammell indicated that previously employed full-wave rectification of a received pulse echo, followed by signal smoothing employing a filter, hindered the resolution of closely spaced tissue interfaces. Gammell indicated that when an electronic signal was treated as an analytic signal, the real signal was replaced by a complex form of both real and imaginary parts. Gammell stated that it was known that the analytic signal correlated to the rate of arrival of energy, i.e., that the square of the magnitude of the analytic signal was proportional to the instantaneous rate-of-arrival of the total energy of the reflected signal. He contrasted this result to the fact that the square of the real signal, was proportional to the rate of arrival of only one of the components of the energy. As a result, the square of the real signal was zero in any instant when one of the component energies was zero, whereas the square of the analytic signal magnitude was only zero when the total instantaneous energy was zero. As a result, use of the analytic signal magnitude enabled optimal estimation of interface location due to the more accurate representation of instantaneous returned energy. However, Gammell employed the analytic signal in lieu of rectification of the ultrasound signal.
To determine analytic signal magnitudes, Gammell took advantage of certain symmetry properties of the Fourier transform of analytic signals. The Fourier transform of a complex ultrasonic signal can be obtained from the Fourier transform of one of its components, by suppressing the negative frequency contributions. The Fourier transform of a real signal is always a symmetrical function, with a total frequency bandwidth of twice the central Fourier frequency. Therefore, the obtaining of the full complex analytic signal was performed by first Fourier transforming the real echo data using a complex fast Fourier transform procedure. Then, all negative frequency components of the Fourier spectrum were set equal to zero and an inverse fast Fourier transform was performed to reconstruct the full analytic signal. The magnitude of the analytic signal was then calculated as the square root of the sum of the squares of the real and imaginary parts at each point in time.
Lizzi et al. further combined signal deconvolution and analytic signal processing to further enhance echo signal data--See "Ultrasonic Ocular Tissue Characterization" pages 41-61, especially pages 45, 46 in "Tissue Characterization with Ultrasound" Greenleaf, editor, CRC Press, 1986.
Thus, to summarize the known prior art, Gammell disclosed that the analytic signal was useful for increasing the resolution of tissue interfaces. Gammell indicated that the analytic signal and its analysis should be used as an alternative to rectification and not in addition thereto. Lizzi et al. taught that ultrasonic tissue characterization could be improved by deconvolving a received scan to remove system-created anomalies and further processed the scan by deriving an analytic signal therefrom.
Notwithstanding the above described prior art echo enhancement procedures, advances in refractive surgery, and in particular excimer laser corneal ablation, provide the surgeon with an ability to remove corneal tissue layers with sub-micron accuracy. Considering that the corneal epithelium has an approximate thickness of 50 microns, it is easily understood that local epithelial thicknesses must be accurately assessed to assure that the laser ablation mechanism is properly controlled. This requires higher levels of resolution of corneal layers than has been heretofore achieved.
Accordingly, it is an object of this invention to provide an improved ultrasound imaging system, particularly adapted to imaging of corneal structures.
It is a further object of this invention to provide an improved ultrasound instrument, having a substantially improved resolution capability.